fix(sla): defer SLA audit events until after transaction commit

[Magrexy]

Summary

Closes #236

Why

SLA audit events were logged inside compute_device_sla() before the caller's transaction committed. Since the audit log uses its own database session, these events were persisted immediately — even if the SLA computation failed to commit. This left orphaned audit records describing work that never occurred.

What was built

File	Change
app/services/sla_service.py	Removed record_sla_settlement_audit_events() calls from the success path of compute_device_sla() (both no-outages and with-outages branches). The exception-handler audit log for failures is kept since failures are independent of transaction state.
app/tasks/sla_tasks.py	Added record_sla_settlement_audit_events() calls after _mark_success() — the point where the SLA computation is committed to the database.
tests/test_sla_audit_deferred.py	6 new tests: verifies compute_device_sla() never emits audit events, verifies the failure path still logs, and smoke-tests the record_sla_settlement_audit_events() helper.

Integration changes outside <module>/

No new env vars, feature flags, or imports introduced. Existing record_sla_settlement_audit_events() function signature is unchanged.

Acceptance criteria coverage

• Audit events are only logged after the transaction commits (success path moved to sla_tasks.py after _mark_success())
• No orphaned audit records when SLA computation fails (exception-path audit log kept inside compute_device_sla())
• Tests verify compute_device_sla() never emits audit events directly (test_no_outages_does_not_emit_audit, test_with_outages_does_not_emit_audit)

Deliberately deferred

• Audit log for the failure path in compute_device_sla() remains inside the function — this is intentional since it doesn't depend on transaction state

Test plan

• pytest tests/test_sla_audit_deferred.py -v — 6/6 passing

Env vars / Notes

No new env vars. No migration required.

[usmanimamu17-create]

LGTM. thanks for contributing.